1. Field of the Invention
The present invention relates to a combination torsion spring and a shift mechanism including the combination torsion spring.
2. Description of the Related Art
Torsion spring (torsion coil spring) 1 (see FIGS. 1A through 1C) that receives a torsional moment (torque) around a winding center axis (coil axis) of a coil section that is formed by winding a steel wire into a coil moves and stops a mechanism of a mechanical component and a mechanism of an electronic instrument. Consequently, torsion spring 1 is used in various ways as, for example, a shock absorber when storing or taking out a vehicle seat or the like as described in Japanese Patent Application Laid-Open No. HEI 6-144092, or a locking apparatus that fixes a display device at an arbitrary position as described in Japanese Patent Application Laid-Open No. HEI 10-222079.
In general, when torsion spring 1 shown in FIG. 1A receives a load in a direction in which hook sections 3 that are formed by bending both ends of coil section 2 to separate from or approach each other in a circumferential direction, as shown in FIG. 1B, a rotational moment M is generated in coil section 2 of torsion spring 1 in accordance with the magnitude of the load acting on hook sections 3. Therefore, in torsion spring 1, from the state shown in FIG. 1B, winding center axis L of coil section 2 inclines relative to the position of central axis L in FIG. 1B, as shown in FIG. 1C. As a result, since the relative distance between hook sections 3 changes in a direction that decreases the loads acting on hook sections 3 (loads in D1 and D2 directions that open to the left and right in FIG. 1B, that is, loads in directions away from each other that are perpendicular to the coil axis that act on hook sections 3), there is the problem that a predetermined reaction force cannot be obtained from torsion spring 1.
Further, when torsion spring 1 receives a load in a direction in which hook sections 3 at both ends of coil section 2 separate from or approach each other in the circumferential direction, stress concentrates at bent sections 3a of hook sections 3, and most of the bending stress in the torsion spring 1 arises at bent sections 3a of hook sections 3. Therefore, from the viewpoint of repeated bending fatigue of the spring material, in general, an upper limit of a load that actuates torsion spring 1 is set by taking into account the bending stress that acts on bent sections 3a of hook sections 3. In other words, since the working load of torsion spring 1 is restricted by the bending stress that acts on bent sections 3a of hook sections 3, in some cases the working load thereof cannot be freely set to a larger amount even if a margin exists with respect to the amount of stress that acts on coil section 2 that is the main body of the spring.